use hidden visibility for C entry point function _start_c

the file-level crt_arch.h asm fragments generally make direct
(non-PLT) calls from _start to _start_c, which is only valid when
there is a local, non-interposable definition for _start_c. generally,
the linker is expected to know that local definitions in a main
executable (as opposed to shared library) output are non-interposable,
making this work, but historically there have been linker bugs in this
area, and microblaze is reportedly still broken, flagging the
relocation for the call as a textrel.

the equivalent _dlstart_c, called from the same crt_arch.h asm
fragments, has always used hidden visibility without problem, and
semantically it should be hidden, so make it hidden. this ensures the
direct call is always valid regardless of whether the linker properly
special-cases main executable output.

crt/crt1.c

@@ -11,7 +11,7 @@ weak void _fini();
 int __libc_start_main(int (*)(), int, char **,
 	void (*)(), void(*)(), void(*)());
 
-void _start_c(long *p)
+hidden void _start_c(long *p)
 {
 	int argc = p[0];
 	char **argv = (void *)(p+1);